Method of coating aluminum



Feb. 20, 1962 c. L. GOODZEIT ETAL METHOD OF COATING ALUMINUM Filed June 12, 1957 CWJTANT- LEVEL FEED TANK IN VEN T OR? A rroeus Y United States Patent ()fiice 3,022,193 Patented Feb. 20, 1962 3,022,193 METHOD OF COA'IZNG ALUMINUM Carl L. Goodzeit, Birmingham, and Howard W. Pearsall, Highland Park, Micln, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed June 12, 1957, Ser. No. 665,282

' 4 Claims. (Cl. 11750) This invention relates to the application of metal coatings to aluminum and aluminum alloy surfaces involving a method which more specifically is particularly suitable for the application of lead-base alloys to bearing surfaces of aluminum and aluminum-base alloys and an apparatus which can be utilized therewith.

Thick layers of Babbitt linings on bearings have proved to have comparatively poor resistance to fatigue. However, the fatigue strength of the Babbitt lining increases as the thickness of the layer decreases. A Babbitt lining of approximately 0.0007 inch has adequate strength for bearing use. Such a thin Babbitt layer must be supported by a backing material which is particularly strong and resistant to fatigue and seizure.

Aluminum and aluminum alloys, hereinafter referred to as aluminum, which are in turn supported on a steel backing materiaL'are well suited as backing materials for Babbitt linings. Backing members formed of such metals are light in weight, yet have adequate strength and sufficient resistance to fatigue and seizure. The use of aluminum-base backing materials is particularly advantageous due to the good bearing properties of aluminum itself. Thin of theBabbitt metal alloy with the aluminum surface to provide a bearing of superior quality. Aluminum-base alloys which possess particularly good bearing qualities include an alloy consisting essentially of 0.3% to 11.0% silicon, 0.05% to 5% cadmium and the balance aluminum, and an alloy consisting essentially of 0.05% to 3% magnesium, 0.3% to 11% silicon, 0.05% to 5.0% cadmium, 0.1% to 2.25% chromium and the balance aluminum.

In the past smooth, adherent coatingsof Babbitt metal on steel backing members have been obtained by electrodepositing Babbitt metal directly onto the steel backing. Electrodeposition has heretofore been the most satisfactory commercial method of applying Babbitt to an aluminum backing material. Unfortunately, due to the rapid formation of an oxide coating on aluminum surfaces, it is exceedingly difficult to obtain electrodcposition of Babbitt metal directly onto aluminum. Therefore, the aluminum surface requiresextensive pro-treatment before receiving the babbitt. The backing member normally is first immersion-coated with zinc, and then given a. flash coating of copper, the Babbitt metal being electrodeposited onto the flash copper layer. A thin layer of tin is then usually electrodeposited onto all the surfaces of the bearing. This method of applying Babbitt to an aluminum backing material is relatively costly. In addition, the-presence of zinc and copper in the overlay of the aluminum surface, although necessary for adherence of the electroplated Babbitt, has proved in some instances to be detrimental to bearing performance.

Itgis, therefore, a principal object of this invention to provide a method for directly coating aluminum by means of a fused chloride or bromide of a metal below aluminum in the electromotive series. Another object of this invention is to provide a method of joining aluminum parts using such a coating. More specifically, another object comprehended by this invention is to provide an apparatus and process whereby aluminum bearing backing members may be directly coated with a thin layer of Babbitt metal. I

Other objects, advantages and features of the present In accordance with the invention, the aluminumsurface is covered with a layer of a powdered chloride or bromide of a metal below aluminum in the electromotive series, and the composite article is thereafter heated to the fusion temperature of the salt. More particularly, We have employed chlorides and bromides of lead and tin to provide satisfactory results in an expeditious manner. At the temperature required to fuse the salt, a displacement reaction occurs wherein the metal of the salt is reduced to the pure metal and deposited on the aluminum surface. The chlorine or bromine of the salt combines with the aluminum to form a volatile aluminum halide which escapes by vaporization. The fusion temperature of some otherwise satisfactory salts is often undesirably high, such as that of lead chloride which is about 934 F. However, the temperature required to fuse this salt may be reduced to about 750 F. by employing an approximately eutectic mixture of the desired metal salt and other suitable salts, such as potassium chloride and sodium chloride. An example of such a mixture is one consisting of about 68% plumbous chloride, 18% potassium chloride and 14% sodium chloride.

It has been found that the addition of the potassium chloride and sodium chloride not only reduces the temperature required to melt the desired salt but also promotes wetting of the aluminum surface, thereby assisting in the formation of a more adherent coating of the desired metal. In connection with this latter function, it is often desirable to add a small amount of these salts in excess of that required to form a eutectic mixture. For example, in coating aluminum with a mixture of metals, as noted herein-after, small excess amounts of sodium and potassium chlorides do not significantly increase the fusion temperature yet provide desirable additional wetting action. The potassium and sodium chlorides, which do not form part of the coating, rise to the surface/of the coating during fusion, and subsequently can be washed therefrom with water after cooling of the coated article.

If it is desired to coat an aluminum surface with a combination of metals, this end may be accomplished by employing a mixture of the appropriate metal chlorides or bromides, e.g., PbCl and SnCl Coatings composed of two or more metals also can be applied by employing a mixture of a powdered pure metal and a metal chloride or bromide. The mixtures tin powder and PbCl as well as lead powder and SnCl have efficaciously been employed to practice this modification of the invention. The hereinbefore mentioned eutectic forming salts can also be employed in the mixture to apply these coatings when it is desirable to utilize the lower melting point and the wetting effect produced by the potassium and sodium chlorides.

It has been found that such a powdered mixture as described above cannot be conveniently applied to an inclined surface. However, if a small amount of an organic polymer with adhesive properties, such as nitrocellulose or acrylic resin, is incorporated in the powdered mixture, the resulting mixture will adhere to an inclined surface as firmly as to a horizontal surface.

The present invention also comprehends coating aluminum by immersing it into a bath of the molten metal on which the molten chloride or bromide salt of that metal is floating (e.g., lead and PbCl The aluminum is preferably preheated and dipped into this bath, whereupon the metal (e.g., lead) deposits onto the aluminum.

'The melting point of the halide salt can be suitably low- 'ered,'as previously indicated, by employing a eutectic J scribed is suspended in a highly volatile liquid, forming a'slurry, and sprayed onto a heated aluminum surface.

The. volatile liquid rapidly evaporates from such a surface depositing thereon a film of the powdered mixture which is so adherent that it will even cling to vertical surfaces without the use of an adhesive.

The slurry is generally comprised of a metal halide and a powdered metal suspended in a highly volatile or ganic liquid. One such mixture which has been used .to coat babbitt on the aluminum bearing is as follows:

56.5% lead chloride 11.8% sodium chloride 15.0% potassium chloride 12.5 cuprous chloride 4.5% tin powder The above dry ingredients are passed preferably through a 200 m'e'sh'screen, mixed together in a ball mill with a suitable volatile liquid, such as lacquer thinner, in a proone-half parts of lacquer thinner.

as the volatile liquid evaporates, the solids of the slurry are deposited on the bearing surface. This layer of'solids surface. The positioning of the bearing sleeves at the top of the column is aided by afrusto-conical skirt 24 projecting upwardly from the opening 19 of the column 10. The alignment and positioning of the bearing sleeves within the column may be assisted, by indentations 21 on the inner surface of the'column and suitably positioned tangs 23 as shown in FIGURE 2 on the outer surface of the sleeves which engage the indentations -21 of the column. a V v The bearings progress downwardly throughthe column under force of gravity, the rate of such progression being determined by the speed of an automatic bearing support and take-off or removal device 25 located at the bottom can be built up to a substantial thicknessfthrough successive sprayings of slurry on warm bearings, each layer.

being permitted to'dry before the succeeding layer is applied. thicker metallic coatings after fusion of the mixture.

' Occasionally, a particular coating mixture does not' produce a satisfactory metal deposit when the coating mixture has been built up to a substantial thickness. It

was found that incorporation of an organic adhesive, for'example nitro cellulose lacquer, in such coating mixtures permitted thicker deposits of the metallic. coating to be prepared which were suitable for bearing use. When' an adhesive was included in the following composition, it permitted a substantial increase in the thickness of the coating mixture which could be applied to produce a satisfactory thicker metallic coating:

10% sodium chloride 10% potassium chloride 70% plumbous chloride 8% tin (powder)! 2% copper (powder) 7 This mixture is passed preferably through a 200 mesh screen and combined with approximately 12 parts adhesive to 100 parts of the above mixture by weight. Suificient lacquer thinner is then added to comprise a final slurry'of about 5% to 10% lacquer thinner.

' Referring more particularly to the drawing, the apparatus shown can be used to coat bearings in accordance with the invention. The apparatuscomprises a cylindrical vertical column 10 of any suitable refractory material. This column includes a bearing spray section 12,

' bearing firing section 14, and a bearing cooling section 16. Semi-cylindrical bearing sleeves 18 are introduced .in pairs in the column through the upper opening 19.

Two conveyors 20 and 22, arranged generally oppo site one another at the upper opening 19 of the column 10, carry the bearing sleeves to the top of the column. The semi-cylindrical bearing sleeves are placed in the top of the column with their inner cylindrical surfaces facing one another, their ends being in general abutment,

thereby forming a generally cylindrical inner bearing Thus, it is possible to obtain correspondingly of the column. A downwardly depending second frustoconical skirt 26 also can be employed at the bottom of the column to assist in the proper positioning of the bearing as it emerges from the lower opening 27. The bearings can be supported at the lower end of the vertical column by any suitable device which will also permit the lowering of the bearings in the column and a removal therefrom. A preferred form for such a device is the toothed wheels 28' and 30 located at the lower end of the vertical column as shown. The semi-cylindrical bearing sleeves are supported by teeth 31 on the outer periphery of the wheels and automatically lowered as the wheels revolve. An adjacent tooth 33 on the wheel then engages the next bearing sleeve to separate it from the one above it and indoing so projects underthe lower edge of the bearing thereby supporting it. v

A rotatably mounted air'tube 32 for introducing dry air into the interior of the column is positioned within the upper part of the column coincidentally with the axis of same. Connected to the lower extremity of the tube 32 is a hollow metal disk 34 which has an outer diameter slightly smaller than the inner diameter of the bearings, thereby permitting free passage of the bearings down the column. The interior of thehollow metal disk communicates with the interior of the tube 32. The upper and lower walls of the 'disk 34 are provided with aperturesfio, forming inlet openings, through which dry forming an upper wall for the annular space 46 defined by the tubular members. A lower wall for the annular space 46 is formed by sealing the lower end 50'of the outer tube to the wall 44 of. the air tube 32. The lower end of the outer tube 40 can be sealed in any suitable manner capable of withstanding the heat of the column and the detrimental efiect of vapors present in the column, a preferred seal. being obtained by welding. Its ends being thus sealed, the annular space 46 defined by the tubular members forms an annular chamber.

The coating slurry, a suspension of metal chlorides or bromides and a powdered metal in a highly volatile liquid,

'is introduced under pressure by a suitable pump '52 into the annular chamber 46 through the'cover 48.. Several small vertically aligned orifices 54 in the outer tubes are located adjacent its lower end 50. The slurry ofthe coating mixture is sprayed under pressure through these orifices onto the inner vertical surfaces of the bearings.

The outer tube 48 and the air tube 32 are rotated by means of a motor 56 communicating with a spur gear. 58 secured to the outer periphery of an upper part of the outer tube. As the slurry mixture is pumped into the slurry chamber, the tubesare rotated and theslurry is sprayed under pressure from the orifices 54 while the tubes are rotating. The bearing sleeves 18 progressing down the tube are therefore coated with the slurry by means of the rotating spray.

Surrounding the bearing spray section 12 of the column is a suitable heating device, preferably a spiral electric resistance heater 60. The resistance heater 60 is connected to an appropriate source of electrical current (not shown) so that the upper portion 12 of the chamber may be warmed to approximately 300? to 400 F. Around the bearing firing section 14 'of'the column 10 beneath the upper warming heater 60 is another heater, preferably spiral electric resistance heater 62 which when connected to a suitable source of electrical current (not shown) is capable of raising the interior temperature of'the encircled portion 14 of the column to approximately 725 to' 825 F. Immediately below the bearing firing section is the bearing cooling section 16 of the column. Surrounding this portion of the column is a cooling coil 64 which is connected to a source of coolant (not shown), preferably water. It is desirable that the cooling section of the column reduce the temperature of the bearing to approximately 400 F. or less before it emerges from the column.

The bearing sleeves 18, prior to coating with the Babbitt metal, are degreased and cleaned in a conventional manner and thereafter dried. 'The bearings are then placed on the conveyers 20 and 22 which introduce the semicylindrical bearing sleeve halves 18 into the top of the column where they are warmed to about 350 F., preferably in the presence of dry air, although either nitrogen or incompletely dried air is also fairly suitable.

The inlets 36 for admission of the dry air into the chamber are located between the spray section 12 and the firing section 14 so that the combustible volatile liquid vapors are blown upwardly out of the column and the corrosive aluminum chloride vapors formed in the firing section are carried out the lower opening 27.

As the bearings progress down the column, they are warmed by the heating coil 60 around the outside of the column. The warmed bearings are then coated with a suitable slurry mixture by means of the rotating spray. During the progression down the column, the caking and building up of the coating mixture correspondingly progresses due to the action of the rotating spray.

The bearings are then subjected to the heat of the firing section 14 wherein the temperature is raised to approximately 800 F. The exact temperature preferred for the firing section will depend upon the melting point of the coating mixture employed. It is advantageous to have the temperature slightly above the melting point of the salt mixture but not high enough to cause any damage to the aluminum. Generally, the mixtures employed for coating lead-base alloys will have a melting point between approximately 725 F. and 825 F. After the bearing has been fired for a sufficient length of time to complete reduction of the Babbitt metal, the bearing progresses to the cooling section 16 of the column which reduces the temperature of the bearing to generally 400 F. or less. Cooling of the bearings in dry air prevents corrosion, which would occur if the hot bearings were introduced into moist air. The cooled bearing is removed from the column and preferably washed in a suitable soap solution. Simultaneous scrubbing with a fiber brush also aids in removing any excess metal salts still remaining in the Babbitt coating.

The present method of bonding a metal to an aluminum surface may also be employed as a means to join aluminum surfaces to each other. Coating mixtures, such as those previously described, may be placed between two aluminum parts, the parts secured together and the assembly heated to fuse the coating mixture. Particularly adaptable to the joining of aluminum parts is a coating mixture comprising plumbous chloride and a suflicient quantity of powdered zinc to produce an alloy bonding material having generally equal parts by weight of lead and zinc.

As hereinbefore indicated, it is .not always convenient to employ a dry powdered mixture for the bonding, for example, when coating an inclined aluminum surface. ,In such instances, it is advantageous to apply the coating mixture as a paste or slurry which is formed by adding to the powdered mixture a suitable amount of a volatile liquid such as water, alcohol, lacquer thinner, etc. Suitable salts of other metals below aluminum in the electromotive series can also be included in the mixture if it is desirable to employ a solder of a different composition than specified above. It is understood that eutectic mixtures, such as hereinbefore described, can also effectively be employed in joining aluminum parts by this method.

Although the present invention has been described in conjunction with preferred embodiments thereof, it is understood that variations and modifications may be found which are obvious to those skilled in the art. I:- is not intended that these preferred embodiments be a limitation of the scope of the invention except as limited by the appended claims.

We claim:

1. The method of making an aluminum backed multilayer bearing which comprises heating an aluminum hearing member while progressively lowering said bearing member within a vertical column, providing a dry air atmosphere within said column, successively spraying a slur-. ry onto said heated bearing member as said bearing member progresses downwardly through said column, said slurry containing a volatile carrier and a eutectic mixture of potassium chloride, sodium chloride and at least one metal salt from the group consisting of metal chlorides and metal bromides and said slurry further containing at least one powdered metal wherein said metal in said metal salt and said powdered metal are each below aluminum in the electromotive series, evaporating the carrier in said slurry and depositing contents thereof on said heated aluminum surface, further lowering said bearing members in said column into a region of high temperature sufiicient to fuse said metal and said metal salt to form a metal coating on said bearing member, subsequently lowering said bearing member in said column to a region having a temperature below approximately 400 F., removing said coated bearing member from an opening in the lower end of said column, and washing said metal coated hearing member to remove residual surface salts.

2. The method of making an aluminum backed multilayer bearing which comprises heating an aluminum bearing member while progressively lowering said bearing member within a vertical column, providing a selected atmosphere within said column, successively applying a fluid onto said heated bearing member as said bearing member progresses downwardly through said column, said fluid containing a volatile carrier and at least one metal salt lirom the group consisting of metal chlorides and metal bromides, said metal in said metal salt being below aluminum in the electromotive series, evaporating the carrier in the fluid applied to said aluminum surface and depositing said salt in a solid coating thereon, further lowering said bearing member in said column into a region of high temperature sufiicient to fuse said metal salt to form a metal coating on said bearing member, subsequently lowering said bearing member in said column to a region having a temperature below approximately 400 F., removing said coated bearing member from an opening in the lower end of said column and wasmng said metal coated member to remove residual salts.

3. The method of coating an aluminum member which comprises heating an aluminum member while progressively lowering said member within a vertical column, providing a selected atmosphere within said column, successively applying a fluid onto said heated member as said member progresses downwardly through said column, said fluid containing a volatile carrier and at least one metal salt from the group consisting of metal chlorides and metal bromides, said metal in said metal salt being form a metal coating on'said member, subsequently lowen'ng said member in said column to a region having a temperature below approximately 400 F., removing said coated member from an opening in the lower end of said column and washing said metal coated member to remove residual salts.

4. The method of coating an aluminum member which 7 comprises heating an aluminum member while progressively moving said member within a substantially closed housing, providing a selected atmosphere within said housing, successively applying a fluid onto said heated member as said member progressesthrough said housing, said fluid'containing a volatile carrier and at least one metal salt from the group consisting of metal chlorides and metal bromides, said metal in said metal salt being below aluminum in the electromotive series, evaporating the carrier in the fluid applied to said aluminum surface and depositing said salt in a solid coating thereon, further 10 j if moving said member in said housing into a region of hgh temperature sufficient to fuse said metal salt to form a metal coating on said member, subsequently moving said member in :said column to a region having a temperature below approximately 400 F., removing'said coatedrnemher from, an opening iin the housing and washing said metal coated member to remove residual salts.

lleferences Cited in the file of this patent '1 UN IT ED STATES PATENTS 675,584, Betts June .4, 1901 2,104,370 r 'Johansson 1311,74, 1938 2,348,224 Pelosi May 9, 1944 2,398,738 Gilbert Apr. 16, 1946 2,480,498 I Miller et a1 -l. Aug. 30, 1949 2,483,605 Abramson Oct. 4, 1949 2,667,433 Gebhardt et a1. Jan. 26, 1954 2,734,003 Alpert .a Feb. 7, 1956 2,737,463 Lawton ,et a1. Mar. 6, 1956 Eubanl; Feb. 3, 1959 

1. THE METHOD OF MAKING AN ALUMINUM BACKED MULTILAYER BEARING WHICH COMPRISES HEATING AN ALUMINUM BEARING MEMBER WHILE PROGRESSIVELY LOWERING SAID BEARING MEMBER WITHIN A VERTICAL COLUMN, PROVIDING A DRY AIR ATMOSPHERE WITHIN SAID COLUMN, SUCCESSIVELY SPRAYING A SLURRY ONTO SAID HEATED BEARING MEMBER AS SAID BEARING MEMBER PROGRESSES DOWNWARDLY THROUGH SAID COLUMN, SAID SLURRY CONTAINING A VOLATILE CARRIER AND A EUTECTIC MIXTURE OF POTASSIUM CHLORIDE, SODIUM CHLORIDE AND AT LEAST ONE METAL SALT FROM THE GROUP CONSISTING OF METAL CHLORIDES AND METAL BROMIDES AND SAID SLURRY FURTHER CONTAINING AT LEAST ONE POWDERED METAL WHEREIN SAID METAL IN SAID METAL SALT AND SAID POWDERED METAL ARE EACH BELOW ALUMINUM IN THE ELECTROMOTIVE SERIES, EVAPORATING THE CARRIER IN SAID SLURRY AND DEPOSITING CONTENTS THEREOF ON SAID HEATED ALUMINUM SURFACE, FURTHER LOWERING SAID BEARING MEMBERS IN SAID COLUMN INTO A REGION OF HIGH TEMPERATURE SUFFICIENT TO FUSE SAID METAL AND SIAD METAL SALT TO FORM A METAL COATING ON SAID BEARING MEMBER, SUBSEQUENTLY LOWERING SAID BEARING MEMBER IN SAID COLUMN TO A REGION HAVING A TEMPERATURE BELOW APPROXIMATELY 400* F., REMOVING SAID COATED BEARING MEMBER FROM AN OPENING IN THE LOWER END OF SAID COLUMN, AND WASHING SAID METAL COATED BEARING MEMBER TO REMOVE RESIDUAL SURFACE SALTS. 